1. Field of the Invention
The present invention relates generally to waste treatment systems. More particularly, the invention concerns waste treatment systems whereby the waste is processed by an apparatus comprising a thermal-chemical reaction chamber and a cooperating dual stage thermal oxidizer.
2. Discussion of the Prior Art
Disposal of waste materials, such as trash and garbage has become a serious concern of industrialized nations. Waste is troublesome not only because it represents something that, as a general rule, cannot be used for any beneficial purpose, but also because it presents hazards to the environment in terms of the space it takes up and the deleterious effects it has on living organisms. For a considerable period, the disadvantages inherent in waste were largely ignored or, at least afforded little weight when a new process or new product that would produce waste was introduced, the benefits to society that the process or product would bestow being considered paramount. Inevitably, however, the increasing volume of waste and the dangerous conditions presented by it forced more attention to be paid to ways of dealing with the material, such that planning for waste treatment often today is an important consideration in the design of a new process or product.
In general, refuse from community and from various types of industrial facilities vary widely in composition, and may include, for instance, sludge from sewage, garbage, plastic scraps, tires and other articles of rubber, scrap wood, oil impregnated rags and refuse oils, all of which are organic, as well as concrete debris and scrap metal. The inflammables among these components range widely in heat of combustion from about 1,200 kcal/kg up to about 7,000 kcal/kg. Consequently, it has been necessary to use a variety of types of disposal facilities for handling each type of material.
It has not been possible to treat all of these types of materials by ordinary combustion methods because offensive odors have been generated as a result of imperfect combustion, the production of components which are extremely corrosive, particularly at high temperature, adherence of fly-ash and the presence of substantial amounts of imperfectly combusted components in the residual ash. Disposal of ash also poses problems such as the scattering of ash dust by means of winds or fouling of water. Moreover, provision must be made for preventing corrosion and damage to the combustion equipment and instruments and to preventing pollution of the environment such as is caused by the gases resulting from the combustion of chlorinated organic materials. The increase in the quantity of scrap vinyl chloride resins is a factor here.
Conventionally, in the course of incineration, gasification is carried out by injecting air and steam prior to incineration. The objective is to convert organic materials from different sources into forms, which will burn uniformly in the manner of coal, wood or charcoal; however, refuse varies so widely in properties that the reaction velocity of gasification also varies strongly. Consequently, the difficulty in effecting complete combustion without harm to the environment has been such as to make the incineration operation uneconomical in many cases.
Presently, perhaps the most common method of waste disposal is the so called landfill method of disposal. However, because of the very large volume of waste that is generated on a daily basis particularly in highly populated areas, acceptable landfill sites are rapidly reaching capacity and new sites have become difficult to find. Accordingly, alternate methods of waste disposal, such as pyrolytic destruction of waste, have been actively considered.
By techniques of pyrolytic decomposition, many types of waste materials can be converted into energy rich fuels such as combustible gases and char, or fuel carbon. Accordingly, several types of devices for pyrolyzing refuse and other waste products have been suggested. Many of these devices have proved unworkable or economically unfeasible. Others, while feasible in concept have been proven to be inefficient and unreliable in continuous operation. Still others, while attractive in theory, have been shown to be too expensive to manufacture, install and operate.
Among the most successful prior art refuse conversion devices are the devices described in U.S. Pat. Nos. 2,886,122; 2,993,843; 3,020,212; and 3,098,458. The present invention constitutes an improvement upon certain of the devices described in these patents.
The pyrolytic process employs high temperature in, most desirably, an atmosphere substantially free of oxygen (for example, in a practical vacuum), to convert the solid organic components of waste to other states of matter, such pyrosylates in a liquid or vapor phase. The solid residue remaining after pyrolysis commonly is referred to as char, but this material may contain some inorganic components, such as metals, as well as carbon components, depending on the nature of the starting waste. The vaporized product of pyrolysis further can be treated by a process promoting oxidation, which “cleans” the vapors to eliminate oils and other particulate matter therefrom, allowing the resultant gases then to be safely released to the atmosphere.
A typical waste treatment system utilizing pyrolysis includes an input structure for introducing the waste; a chamber or retort from which air can be purged and in which pyrolysis processing occurs; and means for raising the temperature inside the chamber.
Systems that rely upon pyrolysis often are designed with principal attention being given to system efficiency. For example, to encourage consistent results from the pyrolytic conversion process, various methods and apparatuses commonly are used to pre-treat the waste before it is introduced into the pyrolytic chamber. These include pre-sorting or separating the waste into constituents on the basis of weight, shredding the material to make it of relatively uniform size and perhaps blending it with other pre-sorted material to promote even distribution of the waste as it is introduced into the retort. Several techniques have been employed to reduce the level of moisture in the waste before introducing it into the machine, because the presence of moisture makes the pyrolytic process less efficient. Such techniques include drying by desiccation or through the application of microwave energy.
Other features often are provided to continuously move waste through the treatment unit while the system is being operated, such as a form of conveyance arrangement. Screw conveyors or conveyor belts oriented at an incline have been used to ramp waste material, in units of a defined volume and at a defined rate of flow, up from a storage bin or pre-treatment assembly at the ground level to a charging hopper at the top of the treatment unit through which waste is metered into the pyrolytic chamber. Screw conveyors, auger screws and worm conveyors all have been used to impel waste through the retort while pyrolysis takes place, again, to encourage predictable results from the process.
The manner in which the retort chamber is supplied with heat energy to sustain pyrolysis also tan affect the efficiency with which the process can be carried out. For example, it has been found that uniform application of heat to the outer wall of the retort, through which it is conducted into the interior of the chamber, reduces the risk that the retort will buckle from uneven distribution of high temperatures and tends to encourage a more even distribution of heat and consistency of temperature throughout the chamber, which leads to consistent processing results. System features provided to address even heating have included those directed to the manner in which the primary source of heat energy, commonly fuel gases, being combusted in a heating chamber, is arranged with relation to the retort, and the number and placement of fuel gas injection ports, etc.
It further has been known to provide a feature which encourages the efficient use of heat to sustain the pyrolytic process, such as one that allows the recycling of gases that have once been combusted to supply heat energy to the pyrolytic chamber back through the gas injection port, where the gases can be ignited again with a fresh supply of oxygen or air.
Efficiency-promoting elements also can be provided for the processing and recycling of off-gases or vapor pyrosylate. For example, it is known that if a pressure gradient is maintained between the retort and the gas processing arrangement in the direction of the exhaust, the vapor pyrosylate naturally will tend to flow into the cleaning elements. To avoid wasting energy, the cleaned high temperature gases can be used to provide energy to some sort of generating station, such as to heat water in a boiler that supplies a steam generator.
What has long been needed and heretofore has been unavailable is an improved pyrolytic waste treatment system that is highly efficient, is easy to maintain, is safe, reliable and capable of operation with a wide variety of compositions of waste materials, is easy to maintain and one that can be constructed and installed at relatively low cost. The thrust of the present invention is to provide such an improved pyrolytic waste treatment system.